1. Field of the Invention
The present invention relates to a method and an apparatus for holding an optical member and the like with great accuracy, which are suitable for holding a projection lens for use in a semiconductor pattern exposure system, for example.
2. Description of the Related Art
Since a projection lens in a semiconductor pattern exposure system is required to have a high resolving power, the accuracy in holding the lens also must be high. The projection lens typically has a great NA (numerical aperture) in order to obtain a high resolving power. Further, in order to expose and transfer a pattern being equivalent to one chip to several chips in one exposure shot, the lens must have a considerably large image-plane size. Therefore, the projection lens is generally formed by a combination of 20 to 30 single lenses to reduce, to the utmost limits, aberration, such as chromatic aberration, distortion, curvature of the field, astigmatism, and coma.
In the computerized, contemporary world, an optical design is performed to make the above-mentioned aberration fall in the range of a targeted tolerance by appropriately changing the spherical shape of each lens, the thickness of each lens, and the spacing between individual lenses. The manufacturing of a spherical surface on individual lenses is conventionally performed by lapping with a so-called lapping dish face-by-face. After both surfaces of the lens have been worked, the outer peripheral surface of the lens is polished so as to be axially aligned with a straight line connecting the centers of two spherical surfaces (i.e., an optical axis).
On the other hand, in the design and manufacture of a lens barrel, the manufacturing precision will depend on how efficiently the position and spacing of each individual lens is maintained according to the design. The performance of a finished lens is identified by residual aberration in the optical design and the manufacturing error.
FIG. 11 illustrates an arrangement for holding a lens (i.e., a lens barrel structure) generally used in a projecting lens. In a lens barrel having a comparatively low accuracy, an arrangement in which the outer portion of the lens is fitted and fixed in the inner diameter of the lens barrel is used (this will be called a first system). The body of a barrel 52 is previously manufactured for assembly, as shown in the drawing. When a single lens 51 is inserted in the lens barrel 52, a portion 53 of the lens barrel 52, which will be in contact with one side of the spherical surface of the single lens 51, is called an abutting portion. The abutting portion 53, corresponding to a single lens 51 having a concave bottom surface, has a cylindrical shape with a hook-shaped cross section. Since a cylindrical external diameter portion of a single lens (called a lens edge portion hereinafter) is manufactured by rotating around the line connecting the centers of the top and bottom spherical surfaces (i.e., along an optical axis), when the lens edge portion is fitted in an internal cylindrical portion of the lens barrel, the cylindrical portion is aligned with the optical axis of the single lens. By fastening the convex surface of the other side of the single lens through a cap ring 54 at a predetermined torque, the single lens 51 is fixed to the lens barrel 52, losing a degree of freedom in the directions of six coordinates axes. Another single lens 55 is also fixed in the same way. When an optical system is formed by additional single lenses, another lens barrel 56 is connected to the lens barrel 52 through a fitting portion and a screw portion.
In another system, a single lens is fixed to a metal ring called a cell to be inserted in a lens barrel using the cell as a reference instead of directly fitting the single lens in the lens barrel (this will be called a second system). The single lens and the cell are connected one to one and as methods for connecting them, a cap ring type, caulking, an adhesive method, etc., are known.
This second system has advantages such as (1) unintelligible coordinates, such as two spherical surfaces, can be converted to understandable coordinates for mechanism and assembling, such as thickness and external diameter, (2) after the single lens is fixed to the cell, the cell can be manufactured relative to (or conformed to fit) the single lens, since it is easier to machine metal than glass, (3) single lenses having different diameters can be fitted into a lens barrel having a single internal diameter by being piled together in sequence by connecting single lenses by cells having the same external diameters, and (4) an aerial spacing can be adjusted by adjusting the thickness of a spacer between the cells.
However, the aforementioned first system has the following problems.
(1) When the lens edge portion 53 is fitted to the lens barrel 52, it cannot be smoothly inserted if the external diameter of the cylindrical portion of the single lens is not a little smaller than the internal diameter of the lens barrel 52. In the Japanese Industrial Standard (JIS), the internal and external diameter tolerances are standardized, corresponding to the fitting diameter and the level of fitting. For example, in a 200-mm fitting for a level in which smooth insertion and dismantling are possible, a nominal value of the internal diameter is 0 to 0.046 mm, and the tolerance of the external diameter is defined as 0 to -0.029 mm. In this case, the maximum clearance between the outer diameter and the Inner diameter will be 75 .mu.m. This value, however, is not sufficient for the requirements for a high resolving lens, demanding an eccentricity of less than 1 .mu.m
(2) One side of the spherical surfaces of the single lens is supported by a peripheral-shaped protruding holding portion of the lens barrel. The protruding holding portion is expected to be in contact with the entire perimeter of the lens spherical surface to hold it equally around the entire perimeter. Therefore, the spherical surface of the single lens is polished with a high accuracy of approximately a quarter to one fiftieth of the wavelength of light to which the lens is exposed. When the surface of the single lens is tentatively assumed to be perfectly spherical, the protruding holding portion must be a perfect circle to be in contact with it around the entire perimeter. The shape of a metal member as shown in FIG. 11, however, cannot be polished and can only be machined with an accuracy of 1 to 0.1 .mu.m as an upper limit. When a single lens is inserted in a lens barrel in this way, the single lens and the protruding holding portion are supported at several unspecified points. This results in a large deformation, which is asymmetrical with respect to the optical axis.
On the other hand, the aforementioned second system has the following problems. The deformation of a single lens due to poor accuracy of the protruding holding portion of the lens barrel is the same as that of the first system. Since the cell must not be stronger than the lens barrel as understood by the cross-sectional structure, the cell itself may be deformed depending on the supporting method.
FIG. 12 illustrates a measured result of the deformation of a single lens caused by the supporting technique. From this figure, a contact point between the single lens and the protruding holding portion can be assumed. The maximum displacement in the measured result is approximately 100 nm, which is beyond the tolerance range of the design. FIG. 13 illustrates a simulated result of deformation when the cell is supported at three equally spaced points. This will result in deformation of the abutting portion as well. When the single lens is inserted in this part, it is easily assumed that the lens will be supported at the three points.